Deen (ディーン Dīn, Dean in some fan translations) is a Sword Fighter from Fire Emblem Gaiden, who is known for his Brave Sword. Similar to Sonia, Deen works for Geyse as a member of the Geyse Pirates despite hating him due to his evil deeds. If Sonia is defeated by Celica, Deen will retreat into Geyse's Fortress. After Geyse's death, he will join Celica's army, noting that he knew that Geyse would have gotten what he deserved eventually. After the war he vanishes, rumored to have gone to a continent to the south in search of new conflicts.

Deen is, overall, a very threatening unit. He starts with high HP, Skill, and Speed and above average Strength. His offense is undoubtedly quite good at the time you obtain him as he only lacks Luck. His defensive stats, on the other hand, are bad, but that is normal for characters of his class.

Half the problem with Deen has absolutely nothing to do with him as a character and everything to do with the fact that you have to pick between him and Sonia. While it's not automatic that you should pick Sonia every time, she has several distinct advantages to choosing her. The first problem for Deen with this choice is that while he starts out with slightly better stats than Sonia, he loses big time in growths(Deen has 120% total growth and Sonia has 180% total growth split evenly across all stats. Only Mycen and Nomah have worse growth than Deen). Another problem arises in that Sonia is easily the single best Mage unit that Celica can obtain, while Deen will wind up being the weakest of the Mercenary class branch after Promotion. The final problem is that by the time you're able to recruit Deen, you will already have 3 Mercenary units available to you, at least one of which you've likely leveled and promoted to Sword Fighter. A unit that is likely roughly as good as Deen, and any of your Mercenary units will beat Deen in the long run, making him largely redundant.

There are, however, some good points to choosing Deen. To begin with, he is in a very strong class with strong base stats. So even with his poor growths, he will still wind up as a good unit. Dread Fighters are deadly, no matter which character promotes to it. Having an army full of them isn't really a negative. And Deen's starting stats are enough to still make him a good unit. The second good thing is that Deen comes with a Brave Sword(which, in Gaiden, acts more like a Killing Edge), whereas Sonia comes with a Steel Shield. While the Steel Shield is usable by everyone, and thus more versatile, it's not quite as potent of an item as the Brave Sword, which has a whopping 30% Critical chance and can turn any Sword wielding unit into a killing machine.

Deen himself isn't a bad unit. On the contrary, he is quite good, particularly when promoted to Dread Fighter, one of the better classes available in the game. it's just that he's shackled to Sonia, who is the best among her Mage peers, while Deen will be the worst among his other, stronger Dread Fighter peers.